1. Field of the Invention
The present invention relates to the washing of semiconductor wafers to remove impurities or particles from the wafers during the manufacturing of semiconductor devices. More particularly, the present invention relates to a semiconductor wafer washing system and to a method of supplying chemicals to the washing tanks thereof to produce a washing solution.
2. Description of the Related Art
Semiconductor devices are generally made by forming at least one circuit pattern on a wafer through repeated and selected processes such as photoresist patterning, etching, washing, diffusion, and metal deposition processes. The washing process is performed to remove various types of particles and impurities that have been produced during the previous processes so that such particles and impurities do not affect the subsequent processes. For instance, the etching away of a patterned photoresist can produce particles of polymer. The wafer is washed after the etching process to remove the particles.
The density, quantity, and temperature of the washing solution must be established and controlled on the basis of various parameters, such as the manufacturing processes involved, the type of wafer, and the like. To meet such requirements, the washing apparatus has various kinds of sensors, including an optical sensor, to provide feedback for the control of the chemical characteristics, the quantity and temperature of the washing solution, etc. The general structure and operation of a conventional washing apparatus will now be described with reference to FIGS. 1 and 2.
A plurality of wafers W are transferred with a cassette K to the apparatus (ST10) where they are positioned in a loading zone 10 (ST12). Then, the cassette K and wafers W are transferred to an aligner 12 in which the wafers are arranged (ST14) with their flat zones (not shown) aligned at a reference position in the cassette. Subsequently, the cassette K and the wafers W are transferred to a first transfer station 16 by a transfer device 14 (ST16).
The first transfer station 16 confirms the number of the wafers W (ST18) and, at the same time, transfers the wafers from the cassette K (ST20) to a robot 18 having a robot chuck (ST22). The cassette K is then transferred by the transfer device 14 to a stand-by area 20 and, after a predetermined period of time, is placed at a second transfer station 26 (ST24)(ST26). The wafers are then successively transferred by the robot 18 into a plurality of washing tanks 22 containing washing solution (ST28).
The wafers W are submerged in the washing solution by the robot so that impurities on the wafers W are washed away by the washing solution. The wafers W are then transferred by the robot 18 to a drying station 24 where the wafers W are dried (ST30). The wafers W are then transferred by the robot 18 to the second transfer station 26. The second transfer station 26 arranges the wafers W again and confirms whether the same number of wafers W that arrived at the first transfer station 16 have undergone the washing process (ST32).
The cassette K is transferred from the stand-by area 20 to the second transfer station 26 prior to the arrival of the wafers W. The second transfer station 26 loads the wafers W back onto the cassette K that has been so transferred from the stand-by area 20 to the second transfer station 26 (ST34). In this way, the wafers W and cassette K are continuously unloaded and transferred to a position in preparation for the next process (ST36).
As was mentioned previously, the conditions of the washing solution in each of the washing tanks 22, i.e., the concentration, quantity, temperature, etc. must be maintained constant according to the type of wafer W or process that dictates the need for cleaning the wafers W. Accordingly, when a new type of wafer is to be washed, a new type of washing solution must be used. It is difficult to create new environments that facilitate the cleaning of new types of wafers with new cleaning solutions. In addition, the old type of washing solution must be discharged out of the system regardless of the remaining useful life thereof, and the washing tanks 22 and washing solution supply 28 must be thoroughly cleaned before they are to accommodate the new type of washing solution. This process of changing the washing solution requires a lot of time.
In addition, the process also suffers from inefficiency because effort must be spent in converting the apparatus to provide for the new washing solution conditions. For instance, the new washing conditions may require additional washing tanks 22 to be provided in the washing apparatus. Due to space considerations, this sometimes requires the removal of existing elements, including the washing tanks 22, from the apparatus and the addition of completely new washing elements. Changing the existing tanks also requires a lot of time.
As an alternative to modifying the washing apparatus, a completely new washing system may be provided in the production line. However, this may require reducing the size of the existing line or the new washing system might be just too large to incorporate into the existing line. In addition, fabricating and installing a completely new washing system is much more expensive in terms of material and labor than merely modifying the existing washing system.
As is also apparent from the description above, the elements of the washing apparatus are arranged in line so that the wafers can be cleaned in a continuous operation. As shown in FIG. 3, the washing solution, which comprises a mixture of chemicals, is supplied to the washing tanks 22 through a pipeline of a washing solution supply 28. The efficacy of the washing solution is reduced as the washing solution undergoes chemical reactions with the impurities on the wafers W. Therefore, the washing solution is discharged out of the system after a predetermined period of time so that pure washing solution can be introduced into the tanks 22. Nonetheless, it is difficult to maintain the required concentration of the washing solution even when a plurality of washing tanks are provided.
Moreover, the period of refreshing the washing solution is relatively short. Thus, the conventional method suffers from a drawback in terms of the costs associated with the amount of washing solution that must be used.